There are a few reasons this may happen:
  1. Pay on arrival orders in Advance sales where the payment was added at a later date than the original order was created.  Please note that the order post date will change to the payment's date, causing the order date and order post date to not match.
  2. Group sales reservations where the payment was added before or after the order was created.  The order date will be the date of arrival for the group, whereas the payment dates will be the date you add the payment to Altru.  
  3. Unresolved Online Sales orders that are completed on a different date than they were rung up online.  If you complete the unresolved order on a day different than when the payment was made, the order date will be different than the payment date.  To see if the order was an unresolved online sales order, go to the Documentation tab of the order, and you will see a Warning Note with the unresolved online error. 
To make the payment's post date match the order's post date, it can be linked to a deposit on the date of the order, which will override the payment's original post date.